[upalvstavai]

Fixes #2822.

Layered portrait coverage was calculated from global_position values. During an editor preview rebuild those global transforms can temporarily be invalid, especially after switching through an empty portrait, so subtracting them propagated inf/NaN into the preview rectangle.

This computes each sprite's transform relative to the portrait root from local transforms instead. It does not depend on the root's global transform being ready, and it also measures all four transformed corners so nested, scaled, rotated, and mirrored layers remain covered correctly.

Added focused GdUnit coverage for:

• a portrait root with an invalid (inf, inf) preview position and a nested sprite;
• a scaled and rotated sprite, verifying the resulting bounds.

Validation:

Godot --headless --path . -s res://addons/gdUnit4/bin/GdUnitCmdTool.gd -a res://Tests/Unit/layered_portrait_coverage_test.gd --ignoreHeadlessMode

Result: 2 test cases, 0 errors, 0 failures, 0 orphans.

The macOS sandbox emitted the repository-documented system certificate warning and could not create user://logs; neither affected the test result. Native Windows editor verification is still appropriate for the original Windows-only interaction sequence.